(1) Field of the Invention
This invention relates to a solid-stage image pickup device for use in a television camera etc. Particularly, it relates to a solid-state imaging device which has a plurality of picture elements disposed in a surface region of a semiconductor body. More specifically, it relates to a solid-state device which has picture elements for reading out from photodiodes photo information stored therein.
(2) Description of the Prior Art
As a prior-art solid-state imaging device, there has been one as shown in FIG. 1 and FIG. 2.
FIG. 1 shows a typical example of the construction of a two-dimensional solid stage imaging device. A photodiode 1 and an insulated-gate field effect transistor (hereinbelow, termed "MOS Tr") 2 make up a picture element as a unit. The array of the picture elements is selected by a horizontal scanning circuit (horizontal scanner) 9 and a vertical scanning circuit (vertical scanner) 10 each of which is a shift register formed of, for example, MOS Tr's. Thus, MOS Tr's 3 and 2 are respectively turned "on" and scanned in succession. By the scanning, charges having been produced by light as stored in the photodiodes 1 are led out from an output terminal 8 through signal lines 5 and 6, and picture signals which the picture elements have received are taken out as electric signals. For some purposes, a plurality of signal lines 6 and output terminals 8 are disposed. Shown at 20 is a voltage source for video output.
FIG. 2 shows a typical sectional structure of the picture element. Hereunder, for the sake of convenience of the description, an N-channel image pickup device in which the signal charges are electrons will be described. The following explanation is quite similarly applicable to a P-channel device by merely inverting the conductivity type and the polarity.
In the picture element illustrated in FIG. 2, the photodiode is formed of a silicon (Si) body 11 made of single crystal silicon of the p-type conductivity and a diffused layer 12 of the n-type conductivity. The n-type diffused layer 12 simultaneously serves as a source, to form the insulated-gate field effect transistor (MOS Tr) along with a gate electrode 13 made of, e.g., polycrystalline silicon, a silicon dioxide (SiO.sub.2) film 16 being thin under the gate electrode 13 and an n-type diffused layer 14 serving as a drain. The n-type diffused layer 14 is usually provided with an electrode 17 of a metal such as aluminum (Al) in order to lower the electric resistance thereof, and the electrode 17 is employed as the signal line 5 in FIG. 1. The SiO.sub.2 film is usually thickened outside the picture element (at 16') in order to suppress the generation of an unnecessary parasitic capacitance.
Upon incidence of light 15, electron-hole pairs are created in the n-type diffused layer 12 and the Si body 11. The electrons in the pairs flow into the n-type diffused layer 12 as the signal charges, and are stored in a pn-junction capacitance 18 between the n-type diffused layer 12 and the Si body 11. When a positive scan pulse is impressed on the gate electrode 13, the electrons are drawn by and to the n-type diffused layer (drain) 14 held in a positive potential and are led out to the output terminal 8.
The potential of the n-type diffused layer 12 consequently becomes a positive potential, which drops because the pn-junction capacitance 18 continues to store electrons developed by the light 15 until the next positive scan pulse is impressed.
Such a prior-art solid-stage imaging device, especially the photodiode being the heart thereof, structurally involves four disadvantages to be described hereunder. Accordingly, notwithstanding that a practical solid-state imaging device is eagerly desired, it is not yet realized.
The first problem concerns the spectral response or the spectral sensitivity characteristics. The absorption of light by silicon differs depending on the wavelength. The absorption factors of red light (wavelength: 0.65 .mu.m), green light (0.55 .mu.m) and blue light (0.45 .mu.m) constituting the primary colors are indicated as R, G and B in FIG. 3, respectively. The blue light at B creates electron-hole pairs in the vicinity of the surface of silicon, while the red light at R penetrates deep into silicon and creates electron-hole pairs. In the surface of silicon, the probability at which the recombination takes place to extinguish the electron-hole pairs is high, and the sensitivity lowers. As a result, the photodiode employing silicon is highly sensitive to only the red light (light of longer wavelength). Further, the number of electron-hole pairs is inversely proportional to the wavelength with respect to an identical energetical intensity of light, with the result that the sensitivity to the light of longer wavelength becomes still higher. Therefore, the spectral response of the photodiode becomes a distorted one in which the sensitivity is higher on the longer wavelength side. Accordingly, in the case where the electric signals obtained from the prior-art imaging device are reproduced into a picture, the picture becomes an unnatural one in which a blue part of an object is blackish and a red part is whitish.
The second problem concerns the impingement of intense incident light. Although the junction capacitance 18 in FIG. 2 is naturally saturated when the light 15 is intense, a large number of electron-hole pairs are created even in a deep part of the silicon body 11 as illustrated in FIG. 3. In the electron-hole pairs, the electrons being minority carriers do not always advance towards the particular n-type diffused layer, but they also diffuse in the lateral direction to be injected into the adjacent n-type diffused layer. As a result, the optical signal spreads among many picture elements. Due to the spreading, not only the resolution is degraded, but also a large bright area appears in the reproduced picture, so that the picture frame is spoiled (this phenomenon is called "blooming"). In the prior art including different types of imaging devices, for example, one employing charge transfer devices (charge coupled devices etc.) in a scanning system, the phenomenon occurs conspicuously.
The third problem concerns uniformity in the device characteristics. Ordinarily, the impurity concentration of the silicon body 11 varies 10% or more locally. This nonuniformity is attributed to the manufacturing process of single crystal silicon. In order to suppress the variation to a small value, it must be determined that the silicon body 11 will become very expensive.
The nonuniformity of the impurity concentration of the silicon body 11 gives rise to nonuniformity in the pn-junction capacitances 18 and nonuniformity in the operating characteristics of the MOS Tr's, and it becomes a cause for conspicuously degrading the quality of the reproduced picture.
The fourth problem is that the picture element needs to be made small in order to enhance the resolution, which inevitably renders the area of the n-type diffused layer 12 small. In consequence, the junction capacitance 18 becomes small, and the quantity of electrons, i.e., signal charges which can be stored decreases. In particular, the area of the n-type diffused layer 14 from which the signal charges are taken out is made small to the utmost irrespective of the size of the picture element, and bearing the strain, the n-type diffused layer 12 is reduced further conspicuously. Moreover, the number of the n-type diffused layers 14 to be connected with the signal line 5 in FIG. 1 increases with the number of the picture elements, and each n-type diffused layer 14 has a junction capacitance 19 (FIG. 2), so that a large capacitance with the junction capacitances combined with parasitic capacitances of the signal lines 5 and 6 develops. As a result of the above, the electric signal appearing at the output terminal 8 is very small, is buried in electric noise and cannot be detected. With the present technology, therefore, a resolution to the extent of that of a television picture cannot be attained.
Ordinarily, this problem can be solved if the junction capacitance 18 is enhanced approximately 1 order. The junction capacitance 18 is increased by raising the impurity concentration of the silicon body 11. Since, however, the junction capacitance 19 increases simultaneously, this method is imperfect.
On account of the problems as described above, the prior-art imaging device has poor resolution, and its working conditions are extremely limited, so that practical use is hindered.
Among the problems stated above, the inadequacy of the spectral response can be compensated by using a pre-filter which absorbs the light of longer wavelength. However, the degradation of resolution for the light of longer wavelength and the development of blooming are unavoidable, and these become serious factors for impeding practical use. In the case of a monolithic solid-state color imaging device based on a method in which filters arranged in the form of a mosaic are superposed on a single imaging device, there is involved a new problem to be solved that the so-called color mixture occurs in which photo electrons created by light incident on, e.g., a diode for red light flow into the adjacent diode for light of another color whereby color information of electric signals to be taken out are mixed.